Hyperpolarization of nuclear spin ensembles has increased NMR sensitivity to a level that is now enabling detection of metabolism in biological tissue on a time-scale of seconds. See, Ardenkjaer-Larsen, J. H.; Fridlund, B.; Gram, A.; Hansson, G.; Hansson, L.; Lerche, M. H.; Servin, R.; Thaning, M.; Golman, K. Proceedings of the National Academy of Sciences of the United States of America 2003, 100, 10158-10163; and Golman, K.; Zandt, R. I.; Lerche, M.; Pehrson, R.; Ardenkjaer-Larsen, J. H. Cancer Res 2006, 66, 10855-60, each of which is incorporated herein in its entirety by reference. The most developed of these technologies, dynamic nuclear polarization (“DNP”, see, Carver, T. R.; Slichter, C. P. Physical Review 1953, 92, 212; Carver, T. R.; Slichter, C. P. Physical Review 1956, 102, 975; and Overhauser, A. W. Physical Review 1953, 92, 411, each of which is incorporated herein in its entirety by reference.), in particular has already been used to detect, grade, and monitor response to therapy in tumors. See, Albers, M. J.; Bok, R.; Chen, A. P.; Cunningham, C. H.; Zierhut, M. L.; Zhang, V. Y.; Kohler, S. J.; Tropp, J.; Hurd, R. E.; Yen, Y. F.; Nelson, S. J.; Vigneron, D. B.; Kurhanewicz, J. Cancer Res 2008, 68, 8607-15; Day, S. E.; Kettunen, M. I.; Gallagher, F. A.; Hu, D. E.; Lerche, M.; Wolber, J.; Golman, K.; Ardenkjaer-Larsen, J. H.; Brindle, K. M. Nat Med 2007, 13, 1382-7; and Golman, K.; Petersson, J. S. Acad Radiol 2006, 13, 932-42, each of which is incorporated herein in its entirety by reference. These encouraging developments have demonstrated the overall viability of NMR based hyperpolarized methods for the study of in vivo metabolism, and are spurring development in alternative methods of hyperpolarization, such as parahydrogen induced polarization (PHIP). See, Adams, R. W.; Aguilar, J. A.; Atkinson, K. D.; Cowley, M. J.; Elliott, P. I.; Duckett, S. B.; Green, G. G.; Khazal, I. G.; Lopez-Serrano, J.; Williamson, D. C. Science 2009, 323, 1708-11; and Bowers, C. R.; Weitekamp, D. P. Phys Rev Lett 1986, 57, 2645-2648, each of which is incorporated herein in its entirety by reference. Polarization yields from the less mature PHIP technology are similar to DNP, and are achieved at significantly reduced instrumental complexity and expense.
Efficient methods for transforming parahydrogen spin order into heteronuclear magnetization at low field in arbitrary spin systems are necessary in particular, for translating emerging contrast agents to biomedical applications. Whereas in DNP polarization is obtained directly on heteronuclei with long lifetimes (at cryogenic temperature), hyperpolarization from PASADENA is captured (at room temperature) in the form of nascent parahydrogen singlet-states, formed upon molecular addition of parahydrogen to an unsaturated carbon-carbon bond. The evolution of this initial ordered ensemble customarily depends on the relative strength of the static magnetic field with respect to the internal scalar couplings. At zero field where chemical shift differences vanish, the combined influence of short and long range scalar couplings lead to a time-dependent dispersion of parahydrogen spin order across the molecule, and therefore an inevitable loss of polarization for any single component of spin active isotopes (e.g. 13C). See, Theis, T.; Ledbetter, M. P.; Kervern, G.; Blanchard, J. W.; Ganssle, P. J.; Butler, M. C.; Shin, H. D.; Budker, D.; Pines, A. Journal of the American Chemical Society 2012, 134, 3987-3990, which is incorporated herein in its entirety by reference. This is an advantage for the emerging applications of high resolution studies at zero field, but is less well suited to in vivo studies because polarization on any single channel of spin-active isotope (e.g. 13C) is exchanged for detailed information regarding long range scalar couplings, which are unnecessary to reconstruct conversion rates across at most two reaction pathways. At high field, the truncation of transverse components in the initial parahydrogen density matrix decreases nominal efficiency by 50%. See, Bowers, C. R.; Weitekamp, D. P. Physical Review Letters 1986, 57, 2645-2648, which is incorporated herein in its entirety by reference. The intermediate strong coupling regime appears to offer a favorable middle ground for PHIP between high and zero fields, where the density operator is retained without truncation and where resonant fields can be used to selectively manipulate spin evolution.
Determining the timing, frequency, and magnitude of these applied fields to efficiently transform parahydrogen spin order into heteronuclear magnetization in the strong coupling regime of protons is nontrivial though, and earlier sequences for application to this field regime have been either been geared towards specific coupling patterns, or have required piecewise or recursive application for optimal results. See, Goldman, M.; Johannesson, H.; Axelsson, O.; Karlsson, M. Magn Reson Imaging 2005, 23, 153-7; and Kadlecek, S.; Emami, K.; Ishii, M.; Rizi, R. J Magn Reson 2011, 205, 9-13, each of which is incorporated herein in its entirety by reference.
Accordingly, a need exists for a single, non-recursive pulse sequence for transferring nuclear singlet state spin order into heteronuclear magnetization localized on a heteronucleus at low magnetic field in the strong coupling regime of protons.
In terms of existing theory, pulsed methods for efficiently converting parahydrogen spin order into net heteronuclear magnetization at low magnetic fields are limited to systems that have only three NMR active nuclei. See, Goldman, M.; Johannesson, H.; Axelsson, O.; Karlsson, M. Magn Reson Imaging 2005, 23, 153-7; and Kadlecek, S.; Emami, K.; Ishii, M.; Rizi, R. J Magn Reson 2011, 205, 9-13, each of which is incorporated herein in its entirety by reference. While raw singlet-states can be long lived and useful in some applications without further manipulation, when applied to biomedicine it is useful to convert these states into net magnetization on a long-lived heteronucleus for storage and to allow subsequent detection using standard imaging techniques. Transforming these states into longitudinal heteronuclear magnetization also maximizes spectral dispersion during subsequent imaging experiments and reduces interference from the intense proton background arising from water in vivo. Furthermore, the initial singlet-state of an AA′XY spin system will generally evolve unless J1X−J1Y−J2X+J2Y=0 and J1X+J1Y−J2X−J2Y=0. Locking magnetization on a heteronucleus therefore makes it unnecessary to synchronize detection with accrued evolution of the initial singlet-state.
Accordingly, a need exists for a single, non-recursive pulse sequence for transferring nuclear singlet state spin order into heteronuclear magnetization localized on a heteronucleus for a system having at least four NMR active nuclei.